As with the others above, I'd like to help, but this just sounds like cheating, not poly. You'd like to retcon it to poly, but I don't think that's possible. Here's an anecdote. I've been open to ENM for a long time. My wife wasn't sure about it, but when we ultimately did it, she asked for it. She had a crush on a coworker and shyly asked me if it would be OK. Then she admitted she had already kissed him. I was a bit surprised but said "Whatever. Go for it." I was still slightly upset that she didn't ask BEFORE the kiss, but it's just a kiss. No big deal. Let's redo that story with your scenario. I would have been hurt. The deception is the problem, and I think you know that.
We all make mistakes, and I'm guessing this is one where at first you thought "Ok, it's a slight transgression" and over time, it got more and more serious and telling your partner got harder and harder. Well, now it's going to be really hard. I'm sorry. We are human, and we make mistakes, but this is a big one. I have no idea how your partner will react, because I don't know him. But your ONLY chance at keeping all the people here is to be honest, work through the repairs, and hope that can restore everything. Alternatives: you break up with one of the two people you love....but which one? My guess is you don't want to lose either. So, it's time to do the hard part.